Transmitter with Quantization Noise Compensation

ABSTRACT

The invention discloses a transmitter comprising a pulse encoder for creating pulses from the amplitude of an input signal to the transmitter, a compensation signal generator for cancelling quantization noise caused by the pulse encoder, a mixer or I/Q modulator for mixing an output of the pulse encoder with the phase of an input signal to the transmitter, said output of the pulse encoder comprising the amplitude of the complex input signal plus the quantization noise caused by the pulse encoder, and an amplifier for creating an output signal from the transmitter. In the transmitter, a control signal (C A ) for controlling a function of the amplifier comprises an output signal from the compensation signal generator, and an input signal to the amplifier comprises an output from the mixer having been modulated to a desired frequency.

RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 13/899,757, filed 22 May 2013, which is a continuation of U.S.patent application Ser. No. 13/320,372, filed 14 Nov. 2011, now U.S.Pat. No. 8,472,557, which is a U.S. National Stage Application (35U.S.C. §371 Application) of International Application No.PCT/SE2009/050552, filed 18 May 2009, the disclosures of which areincorporated herein by reference.

TECHNICAL FIELD

The present invention discloses a transmitter with an increased degreeof efficiency.

BACKGROUND

A transmitter will normally comprise at least one amplifier, theefficiency of which can be enhanced by means of pulsed modulation, i.e.mapping of the modulated communication signal to the characteristics ofa pulse train, such as, for example, duration, position or density. Insuch a transmitter, the pulses are used as input to the amplifier. Thepurpose of driving the amplifier with a pulse train is to make theamplifier operate more of the time at its high efficiency operatingpoints. By using a pulse train, the power amplifier is only operated inone of two highly efficient regions; deep compression or completely off.

However, pulse modulation schemes such as the one described above willintroduce undesired signal distortion caused by quantization noise whengenerating the pulses, which needs to be handled by means of a bandpassfilter, a so called reconstruction filter. Due to the nature of thequantization noise, narrowband filters are needed in order toreconstruct the signal before transmitting. These types of filters havea large insertion loss at microwave frequencies, which leads toincreased power dissipation in the filter, thus reducing the powerefficiency of the transmitter.

Document WO 2006/110590 discloses a power control module which receivesa dynamic power control signal, and generates a differential bias signalwhich is proportional to the dynamic power control signal.

SUMMARY

As previously described, there exists a need for a solution which cansuppress quantization noise in a transmitter which uses pulses as inputto an amplifier in the transmitter. In particular, the solution shouldbe able to suppress such noise in or around a Radio Frequent, RF,carrier, since that is where the noise can cause the most “damage”.

Such a solution is disclosed by means of the present solution in that itdiscloses a transmitter which includes a pulse encoder for creatingpulses from the amplitude of an input signal to the transmitter, acompensation signal generator for cancelling quantization noise causedby the pulse encoder, a mixer or an I/Q modulator for mixing an outputof the pulse encoder with the phase of an input signal to thetransmitter, and an amplifier for creating an output signal from thetransmitter.

In the transmitter of the invention, a control signal for controlling afunction of the amplifier comprises an output signal from thecompensation signal generator, and an input signal to the amplifiercomprises an output from said mixer or I/Q modulator.

In the transmitter, a control signal for controlling a function of theamplifier comprises an output signal from the compensation signalgenerator, and an input signal to the amplifier comprises an output fromthe said mixer or I/Q modulator having been modulated to a desiredfrequency. The transmitter additionally comprises a band pass filter atthe output of the amplifier. In the transmitter, the control signal isat each instant chosen such that the output of the amplifier with thecompensation filter is a linear copy of the input signal to thetransmitter. The function in the amplifier which the control signal isused for controlling being the maximum output amplitude of the amplifieror the impedance of an output matching network comprised in theamplifier.

As will be shown in the following detailed description, the transmitterof the invention is advantageous when it comes to reducing quantizationnoise from the pulse encoder.

In one embodiment, the function in the amplifier which the controlsignal is used for controlling is maximum output amplitude of theamplifier, so called “amplitude modulation”. In one embodiment, thefunction in the amplifier which the control signal is used forcontrolling is the impedance of an output matching network comprised inthe amplifier, so called “load modulation”.

In one embodiment, the transmitter of the invention additionallycomprises a delay circuit for delaying the output of the mixer before itis used as input to the amplifier, in order to compensate for delays inthe compensation signal generator.

In one embodiment, the transmitter of the invention additionallycomprises a digital to analogue converter connected at the output of thedelay circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in more detail in the following, withreference to the appended drawings, in which

FIG. 1 shows a problem which is addressed by means of the invention, and

FIG. 2 shows a prior art solution to the problem, and

FIG. 3 shows a first embodiment of the invention, and

FIG. 4 shows a second embodiment of the invention, and

FIG. 5 shows a result obtained by means of the invention, and

FIG. 6 shows a third embodiment of the invention.

DETAILED DESCRIPTION

FIG. 1 shows a problem which is addressed by means of the invention: asignal x[n], for example in the form of an analogue signal, which can bewritten as x[n]=A[n]*e^(jφ(n)), is used as input signal to a transmitterwhich comprises an amplifier, usually a power amplifier.

Prior to being used as input to the amplifier in the transmitter, theamplitude component A[n] of the input signal x[n], is passed through apulse encoder which “maps” the signal onto an N-bit representation (orin a general case to some discrete signal levels) in order to use thepower amplifier more efficiently. The N-bit representation is usually a1-bit representation, and creates pulses as the output from the pulseencoder.

However, the pulse encoder also introduces an error term, commonlylabelled as quantization noise, which, in the frequency domain, isprimarily centred around the RF-carrier about to be transmitted.

The input signal to the transmitter, x[n]=A[n]*e(n)^(jφ(n)), is shown asx[n] in FIG. 1, and the output signal from the amplifier is shown asy[n] in FIG. 1. As can clearly be seen in FIG. 1, the output alsocomprises quantization noise, shown as q[n], centred around the outputfrequency, so that the output signal can be written as(A[n]+q[n])*e^(jφ(n)).

In FIG. 2, a prior art solution for reducing the quantization noise q[n]is indicated: a bandpass filter, the characteristic of which is shown as“Bandpass” in FIG. 2, can be applied at the output of the amplifier,which will result in an output signal y[n] which is shown in FIG. 2.However, as is also evident from FIG. 2, a residue q[n] of thequantization noise will still remain in the filtered output signal,since a bandpass filter will not be able to have a filter characteristicwhich is sufficiently narrowband to remove all of the quantizationnoise, particularly not the quantization noise which is in the immediatevicinity of the desired signal, i.e. the modulated RF-carrier.

Thus, as also stated previously, it is an objective of the invention toobtain a solution to the problem of quantization noise in the outputsignal of a transmitter with an amplifier which has a pulse-train as itsinput signal.

In the following, the invention will be described by means of threeembodiments of transmitters.

FIG. 3 shows a first embodiment 300 of a transmitter which is based onthe principles of the invention. The transmitter 300 is arranged toreceive a complex-valued input signal, here denoted asx[n]=A[n]*e^(jφ(n)). The input signal is arranged to be fed to a signalcomponent separator 305 which separates the complex valued input signalinto phase and amplitude components. The amplitude component is denotedA[n] and the phase component is denoted as e^(jφ(n)) or exp (jφ[n]).

The signal separator 305 delivers said amplitude component A[n] to apulse encoder 310, i.e. a component which generates pulses as its outputin response to the amplitude of the input signal A[n]. As mentionedpreviously, the pulse encoder 310 will also generate signal distortionin the form of so called quantization noise, so that the output signalfrom the encoder 310 can be written as A[n]+q[n], where q[n] representsthe quantization noise.

The output signal A[n]+q[n] from the pulse encoder 310 is mixed with thephase signal Om from the signal component separator 305 in a mixer 307,so that the output from the mixer 307, here denoted as S[n], can bewritten as S[n]=(A[n]+q[n])*e^(jφ(n)), which can also be written asX[n]+q[n]*e^(jφ(n)). The signal S[n] is then mixed in a mixer 340 with aradio frequency signal, an RF carrier, from a local oscillator, an LO,(not shown in FIG. 3), in order to modulate the signal z[n] to a desiredRF frequency. In some embodiments, the mixer 307 can be replaced by a socalled I/Q-modulator.

The modulated signal is then used as input to an amplifier 320, in orderto amplify the output signal from the transmitter to a desired level.Thus, the input signal to the amplifier 320 comprises an output from themixer 307 which has been modulated to a desired frequency. This is alsothe case for the other embodiments of the transmitter of the inventionwhich will be described in this text.

According to the invention, and as shown in FIG. 3, the amplifier 320 inthe transmitter circuit 300 is arranged to receive as a control signal,C_(A), the output signal from a compensation signal generator 315. Thecontrol signal C_(A) controls a function of the amplifier, in this casethe amplification of the amplifier 320, by means of controlling themaximum output signal amplitude of said amplifier. This can also beexpressed by saying that since the input signal to the amplifier iseither on or off due to the use of the pulse encoder, the resultingamplifier output amplitude for the “on-state” is governed by the controlsignal C_(A).

Turning now to a more detailed description of the compensation signalgenerator 315, the input to this component in the embodiment of theinvention shown in FIG. 3 is two signals, one of which is the amplitudesignal A[n] from the signal component separator 305, and the other isthe output signal from the pulse encoder 310, i.e. A[n]+q[n].

In other words, one of the input signals to the compensation signalgenerator 315 is the “pure” amplitude signal A[n], and the other inputsignal is the output from the pulse encoder 310, i.e. A[n]+q[n]. Asexplained previously, the output from the compensation signal generator315, here denoted as C_(A)[n], is used as control signal to theamplifier 320. Thus, the output signal from the compensation signalgenerator 315 is used to control a function of the amplifier 320, inthis case the amplification of the modulated input signal to theamplifier 320.

A more exact description of the nature and function of the compensationsignal generator 315 is as follows: the compensation signal C_(A) shouldat every instant be chosen such that the output signal of the amplifieris a linear copy of the input signal to the transmitter taking thereconstruction filter into consideration. This can be expressed as:

A[n]e ^(jφ[n])=Σ_(k=0) ^(L) f[k−n]·C _(A) [n]·(A[n]+q[n])e ^(jφ[n])  (1)

where f[k−n] is the impulse response from a filter used at the output ofthe transmitter, a so called reconstruction filter.

The invention's use of the output signal from the compensation signalgenerator 315 as control signal to the amplifier 320 is advantageoussince an extra degree of freedom is introduced in obtaining the finaloutput signal given a reconstruction filter, which will make it possibleto maximize the power efficiency of the transmitter. The RF input signalto the amplifier can be maintained in a so called “deep saturationmode”, thereby ensuring optimum efficiency, while the control signal tothe amplifier will set the correct signal level at the output of theamplifier.

FIG. 4 shows a second embodiment 400 of a transmitter of the invention.Components which have the same basic function as in embodiment 300 inFIG. 3 have retained their reference numbers from FIG. 3.

Apart from the components of the transmitter 300 of FIG. 3, thetransmitter 400 shown in FIG. 4 comprises a delay circuit 430, in orderensure that the signal which is used as input to the amplifier 320 is“synchronized with” the signal which is used to control the amplifier320. Thus, the delay of the delay circuit 430 should suitably correspondto the “processing time” of the compensation signal generator 315, i.e.the delay introduced by the compensation signal generator 315.

The output of the delay circuit, in FIG. 4 denoted as z[n], is putthrough a digital to analogue converter DAC 335, and is then mixed inthe mixer 340 with a Radio Frequency signal, an RF signal, from a LocalOscillator, an LO, (not shown in FIG. 4), in order to modulate thesignal z[n] to a desired RF frequency.

As shown in FIG. 4, the transmitter circuit 400 in this embodimentcomprises a bandpass filter 425, a so called reconstruction filter asmentioned above, at the output of the amplifier 320, in order to removeundesired components in the output signal of the amplifier.

FIG. 5 shows a result obtained by means of the invention: the inputsignal x[n] to the circuit 400 of FIG. 4 is shown in FIG. 5, as well asthe output signal y[n] from the circuit 400. As can be seen, the outputsignal y[n] has a significantly reduced content of quantization noise,q[n], which, as a reference, is also shown in FIG. 5 by means of dashedlines. Thus, the invention reduces the quantization noise in the outputsignal in a sharp area around the “desired” output signal x[n], which isa clear improvement as compared to the solution with a narrow bandpassfilter at the output of the circuit. Remaining quantization noise isshown as q′[n], and as can be seen, q′[n] is located outside of thespectrum of the output signal y[n].

The suppression obtained of the quantization noise by means of theinvention is also shown in FIG. 5 by an arrow “A”, and as mentioned, the“out of band” quantization noise in the output signal is shown in FIG. 5as q′[n].

FIG. 6 shows a further example of an embodiment of the invention.Components which have the same basic function as those in theembodiments 300 in FIGS. 3 and 400 in FIG. 4 have retained theirreference numbers, and as can be seen from FIG. 6, the embodiment 600 isin many ways similar to the embodiments 300 and 400 of FIGS. 3 and 4.Thus, the embodiment 600 comprises a signal component separator 305, apulse encoder 310, an amplifier 320 and a compensation signal generator315, the output, C_(A), of which is used as a control signal to theamplifier 320.

The compensation signal generator 315 of the embodiment 600 has twoinput signals, one of which is the input signal x[n] and the other ofwhich is the output from the mixer 307, again denoted as S[n]. Thus, thetwo input signals to the compensation signal generator 315 in FIG. 5 arex[n] and S[n]=x[n]+q[n]*e^(jφ(n)). The use of the input signal x[n] asone of the input signals to the compensation signal generator 315 isachieved by means of “splitting” the input signal x[n] before thecomponent separator 305, so that one “branch” of x[n] is connected tothe compensation signal generator 315, and the other “branch” of x[n] isused as input signal to the signal component separator 305.

The output S[n] from the mixer 307 is, in the embodiment of FIG. 5, alsosplit, so that one “branch” of S[n] is used as input to the compensationsignal generator 315, as mentioned above, and one “branch” of S[n] isused as input to a first delay circuit 630.

As opposed to the embodiments shown and described previously, thecompensation signal generator 315 of the embodiment 600 produces both afirst and a second compensation signal, here denoted as C_(A)[n] andC_(RF)[n]. The first compensation signal, C_(A)[n], is used as a controlsignal to the amplifier 320, after having been passed through a seconddelay circuit 631. The second delay circuit 630 serves to align (intime) the input signals to the different components.

The output from the first delay circuit 630 is added to the secondcompensation signal from the circuit 315, C_(RF)[n] in an adder 607,with the sum output from the adder 607, denoted in FIG. 6 as z′, beingpassed through a digital to analogue converter 435, following which itis arranged to be modulated by an external LO to a desired RF frequency,and then used as input signal to the amplifier 320. The purpose of thesecond compensation signal C_(RF) is to create further possibilities tofine tune the efficiency of the circuit 600, corresponding to a certainoutput signal from the amplifier.

The combination of the compensation signals C_(A) and C_(RF) should bechosen as follows:

A=[n]e ^(jφ[n])=Σ_(k=0) ^(L) f[k−n]·C _(A) [n]·(A[n]+q[n]−C _(RF) [n])e^(jφ[n])  (2)

The invention is not limited to the examples of embodiments describedabove and shown in the drawings, but may be freely varied within thescope of the appended claims. For example, the control signal C_(A)[n]can also, instead of being used as a means of controlling the outputamplitude of the amplifier by means of supply modulation, be used forcontrolling the impedance of the an output matching network of theamplifier in the transmitter, which is referred to as “load modulation”.In such an embodiment, the matching network is suitably comprised in theamplifier.

What is claimed is:
 1. A transmitter comprising: a pulse encoder forproducing pulses from the amplitude of a complex input signal to thetransmitter; a compensation signal generator operatively coupled to thepulse encoder configured to cancel quantization noise caused by thepulse encoder to generate a first control signal; a first mixer or I/Qmodulator operatively coupled to the pulse encoder for mixing an outputof the pulse encoder with the phase of the complex input signal to thetransmitter, said output of the pulse encoder comprising the amplitudeof the complex input signal plus the quantization noise caused by thepulse encoder; a second mixer operatively coupled to the first mixer orI/Q modulator for converting an output from the first mixer or I/Qmodulator to a desired frequency; an amplifier operatively coupled tothe compensation signal generator and the second mixer and configured toprovide an amplified signal; and a band pass filter operatively coupledto an output of the amplifier, wherein the first control signal isconfigured by the compensation signal generator to control a maximumoutput amplitude of the amplifier or an impedance of an output matchingnetwork comprised in the amplifier such that the output of the band passfilter comprises a linear copy of the complex input signal to thetransmitter.
 2. The transmitter of claim 1, further comprising a delaycircuit operatively coupled to the first mixer or I/Q modulator fordelaying the output of the first mixer or I/Q modulator to compensatefor delays introduced by the compensation signal generator.
 3. Thetransmitter of claim 2, further comprising a digital-to-analog converteroperatively coupled to an output of said delay circuit.
 4. Thetransmitter of claim 1, wherein a first input signal to the compensationsignal generator comprises the amplitude part of the complex inputsignal to the transmitter and a second input signal to the compensationsignal generator comprises the pulses produced in the pulse encoder,inclusive of quantization noise created in the pulse generator.
 5. Thetransmitter of claim 1, wherein a first input signal to the compensationsignal generator comprises the complex input signal to the transmitterand a second input signal to the compensation signal generator comprisesthe pulses produced in the pulse encoder inclusive of quantization noisecreated in the pulse generator multiplied with the phase part of thecomplex input signal.
 6. The transmitter of claim 1, wherein thecompensation signal generator has two inputs used in performingcanceling quantization noise.
 7. The transmitter of claim 1, wherein theamplitude of the complex input signal is indicated by the heights of thepulses output by the pulse encoder.